Numerous electronic devices such as common electronic still cameras and video cameras include solid-state image sensors. A typical solid-state image sensor is formed in a semiconductor chip and includes an array of light-sensitive elements disposed in an area of the front surface of the chip, referred to herein as the “image sensing area.” A color-sensitive image-sensing chip may include arrays of elements sensitive to different wavelengths of light. Each light-sensitive element is arranged to generate an electrical signal representing light falling on a particular small portion of the image sensing area. The semiconductor chip typically also includes internal electrical circuits arranged to convert these signals into a form intelligible to other elements of the device as, for example, into one or more streams of digital values representing the light falling on the various individual pixel areas.
Image sensing chips typically are used in conjunction with optical elements such as lenses which act to focus the image to be observed by the chip onto the active area, as well as wavelength-selective filters. The optical elements most commonly are mounted in a housing referred to as a “turret.” Typically, both the turret and the chip are mounted, directly or indirectly, onto a supporting circuit panel, which supports and electrically interconnects various components of the device in addition to the image sensor. Many image sensor chips are supplied in packages which incorporate a dielectric enclosure surrounding the chip, with a transparent window overlying the image sensing area of the chip. The enclosure is provided with terminals, so that the enclosure can be mounted on a circuit board with the image sensing area and the overlying window facing upwardly away from the circuit board, and with the terminals connected to electrically conductive features of the circuit board. The turret can then be positioned over the package. These arrangements typically require a turret which occupies an area of the circuit board substantially larger than the area occupied by the chip package and substantially larger than the area occupied by the image-sensing chip itself. Stated another way, the area occupied by the turret in a plane parallel to the plane of the image sensing area is substantially larger than the area occupied by the image sensing chip and substantially larger than the area occupied by the package which holds the image sensing chip. This increases the size of the overall device. This problem is particularly acute in the case of very compact devices as, for example, cameras incorporated in cellular telephones and personal digital assistants (“PDAs”).
Moreover, it is important to position the optical elements mounted in the turret accurately with respect to the image sensing area of the image-sensing chip. In particular, to achieve proper focusing of the image on the image sensing area of the chip, it is desirable to position the optical axis of the lenses and other optical elements in the turret precisely perpendicular to the plane of the image sensing area, and to place the lenses at a desired height above the image sensing area. The need for such precise positioning complicates the design of the assembly and, in some cases, may further aggravate the turret size problem noted above.
Another approach which has been suggested is to mount a bare or unpackaged image-sensing chip directly to a turret. In such an arrangement, it would theoretically be possible to achieve good positioning of the chip relative to the optical elements in the turret. However, image-sensing chips are susceptible to mechanical damage and to chemical attack by atmospheric contaminants. Thus, the turret in such an arrangement typically must include arrangements for holding the bare chip in a sealed environment. Moreover, bare imaging sensing chips are extremely sensitive to particulate contamination. As discussed above, each optically-sensitive element provides an electrical signal representing the light falling in a small element of the image, commonly referred to as a picture element or “pixel.” If a particle lands on a particular optically sensitive element, it will block light directed onto that element, so that the resulting signals will show the pixel as dark. When the image is reconstructed from the signals, it will have a dark spot at the affected pixel. Any process which requires assembly of a bare chip with a turret must be conducted under stringent conditions to minimize particulate contamination. Moreover, such processes often suffer from high defect rates caused by particulate contamination. Both of these factors tend to increase the cost of the resulting assemblies. Moreover, these assemblies as well typically require turrets having areas substantially larger than the area of the chip itself.
Thus, there are substantial needs for improved opto-electronic assemblies and assembly methods.